1. Field of the Disclosure
The present disclosure relates generally to improved devices comprising surface-bound ionic liquids for solvating organic compounds and/or common environmental gases (e.g., CO2, H2S). Specifically, the present disclosure relates to piezoelectric gas sensors (e.g., QCM sensors) with bound films of ionic liquids which are capable of detecting volatile organic compounds such as both polar and nonpolar organic vapors and some inorganic gases such as carbon dioxide at both room and high temperatures. In another embodiment, the thin-film ionic liquid provides a basis for the amperometric (e.g., voltammetry) and/or piezoelectric (e.g., QCM) measurement of solvated organic compounds, including volatile explosive organic compounds (e.g., nitroaromatics).
2. Brief Description of Related Technology
Room-temperature ionic liquids are a relatively new class of compounds containing organic cations and anions, which melt at or close to room temperature. An early group of ionic liquids reported by Osteryoung et al. was composed of a mixture of 1-butylpyridinium chloride and aluminum chloride that was liquid at room temperature (Decastro, C., et al., J. Catalysis, 196, 86-94 (2000); and Chum, H. L., et al., J. Am. Chem. Soc., 97, 3264 (1975)). Soon after, a series of ILs based on the cations of alkylpyridinium or dialkylimidazolium were developed. The anions vary from halides, such as Cl−, Br− or AlCl4− to coordinates, such as BF4−, PF6−, SbF6−, or NO3−, SO4−, CuCl2−, and organics, such as CH3SO3−, or (CF3SO2)2N− (Zhao, D. B., et al., Catalysis Today, 74, 157-189 (2002); and Olivier-Bourbigou, H., et al., J. Molecular Catalysis A: Chemical, 182-183, 419-437 (2002)). In the last decade, ILs based on cations of tetraalkylammonium or tetraalkylphosphonium and anions of phosphinate (Robertson, A. J., et al., WO 2002079212; Bradaric, C. J., et al., in Industrial Preparation of Phosphonium Ionic Liquids, ACS Symposium Series 856; Roger, R. D., et al., Edt. American Chemical Society (2003)), alkanesulfonate and alkylbenzenesulfonate (Wasserscheid, P., et al., in New Ionic Liquids Based on Alkylsulfate and Alkyl Oligoether Sulfate Anions: Synthesis and Applications, ACS symposium Series 856, Ionic Liquids as Green Solvents, Progress and Prospects, R. D. Roger and K. R. Seddon Ed., American Chemical Society (2003)) were developed, which are “pure organic” ILs that are more stable, especially at relatively higher temperatures, less toxic and more hydrophobic. Due to its unique properties and increasing availability, room temperature ionic liquids have attracted significant research interest in the past few years.
In contrast to conventional organic solvents that are composed of molecular entities such as DMSO, DMF, CH2Cl2, CHCl3, or THF, ionic liquids have unique properties (Seddon, K. R., in Ionic Liquids for Clean Technology, J. Chem. Tech. Biotech, 68, 315-316 (1997)). They have no significant vapor pressure, thus allowing chemical processes to be carried out with essentially zero emission of toxic organic solvents into the environment. Consequently, they have been considered a possibly environmentally friendly, recyclable media for synthetic organic chemistry, separation sciences and other chemical sciences and engineering (Welton, T, in Room-Temperature Ionic Liquids: Solvents for synthesis and Catalysis, Chem. Rev., 99, 20071-2083 (1999)). For example, ionic liquids have been used as solvents for organic reactions (nucleophilic and electrophilic reactions including acid catalyzed reactions), transition metal catalyzed reactions, and biotransformations (Rogers, R. D., et al., Ionic Liquids: Industrial Application of Green Chemistry, ACS Symposium Series 818, (2002); and Rogers, R. D., et al., Ionic Liquids as Green Solvents: Progress and Prospects, ACS Symposium Series 856 (2002)). In addition to enhanced reaction rates and improved chemo- and regioselectivities relative to other organic solvents, ILs also provide potential solutions for biphasic separation of reaction products via extraction, i.e. products can be obtained through distillation from these non-volatile reaction media which eliminates the need for noxious organic solvents (Visser, A. E., et al., in Task-specific ionic liquids for the extraction of metal ions from aqueous solutions, Chem. Comm. 135 (2001); Bates, E. D., et al., J. Am. Chem. Soc. 124, 926 (2002)). Ionic liquids usually have low miscibility with a number of organic solvents (such as ethers, hexane, or ethyl acetate) as well as supercritical carbon dioxide (Blanchard, L. A., et al., Nature 399, 28 (1999). Consequently, organic compounds can be extracted into supercritical carbon dioxide from ionic liquids.
Ionic liquids possess high ion concentration, high heat capacity and good electrochemical stability. They prove to be excellent candidates for highly efficient heat transfer fluids, supporting media for catalysts as well as electrochemical devices including super capacitors, fuel cells, lithium batteries, photovoltaic cells, electrochemical mechanical actuators and electroplating (Seddon, K. R., J. Chem. Tech. Biotech, 68, 315-316 (1997)). Recently, reports for the use of ILs as lubricants for steels joints (Welton, T., Chem. Rev., 99, 2071-2083 (1999); Rogers, R. D., et al., ACS Symposium Series 818 (2002); and Rogers, R. D., et al., ACS Symposium Series 856 (2002)) show that the ILs exhibits excellent friction-reduction, antiwear properties, both in air and in vacuum, which are superior to phosphazene and perfluoropolyether.
Even though significant progresses in the study of ILs have been made in the past decade, the bulk of current research of ILs is focused on their use as solvents for chemical reactions, separations and electrochemistry. Limited efforts have been made to explore ILs potential for analytical applications (Baker, G. A., et al., in An Analytical view of ionic liquids, The Analyst, 130, 800-808 (2005)). Much fundamental research effort is needed to bring forth the benefits of ILs. There is a need to address this issue and explore ionic liquids surface chemistry and its application as gas sensing materials.
Gas sensors are of increasing interest because of their potential for widespread application in ambient air monitoring, occupational health and safety, biomedical diagnostics, industrial process control, and military and civilian counter-terrorism. Sorptive-polymer interface layers have been extensively explored to temporarily concentrate the vapors near the sensor surface and to facilitate detection by whatever transduction mechanism is employed in the sensing devices (Blanchard, L. A., Nature 399, 28 (1999)). It is now generally accepted that the non-bonding vapor-polymer sorption interactions in sensor arrays do not afford sufficient collective selectivity for quantitative determinations of more than a few vapors simultaneously regardless of the number of sensors or the sensor technology employed (Handy, S.T., Chem. Eur. J., 9, 2938-2944 (2003); Ding, J., et al., Chem. Mater., 15, 2392-2398 (2003); Jensen, M. P., et al., J. Am. Chem. Soc. 125, 15466-15473 (2003); Yang, C., et al., J. phys. Chem. B, 107, 12981-12988 (2003); Barisci, J. N., et al., Electrochem. Commun. 6, 22-27 (2004); Wang, P., et al., J. Phys. Chem. B, 107, 13280-13285 (2003)). Ionic liquids with their unique properties could potentially overcome above limitation for gas detection: (1) ILs are excellent solvents that can support many types of solvent-solute interactions (hydrogen bond, π-π, dipolar, ionic., and the like). Many different interaction types may be simultaneously present in ILs, and the resulting properties of the ILs depend on which interactions are dominant. Consequently, surface design of ILs can be used to fit a particular sensing application; (3) ILs have negligible vapor pressure so that there is no drying out of the electrolyte, which is a serious problem for sensors using solid polymer electrolyte films, which reduces hazards, associated with flash points and flammability; (4) ILs possesses high thermal stability (Liu, W. M., et al., Tribology Letters, 13, 81-85 (2002)). Most ILs show typical decomposition temperatures of 350+° C. This remarkable thermal stability has important implications in the use of ILs for high temperature sensing; (5) Ionic liquids suppress conventional solvation and solvolysis phenomena, and provide media capable to dissolve a vast range of organic molecules to very high concentrations. One of the most exciting and impressive potential industrial applications of ionic liquid is their use for the storage and delivery of gases that are highly toxic, flammable, and/or reactive. Air Products has developed a subatmospheric ionic-liquid-based technology for storing and delivering gases that offers a number of advantages over the solid physical-adsorption technology. This indicates great potential in organic volatile sensing. (6) Synthetic flexibility of ionic liquids allowing them to be tailored to be chemically independent; One ion could be use to deliver one function and the second ion to deliver a different, completely independent function (Wang, H. Z., et al., Wear, 256, 44-48 (2004)). Functionalized ionic liquids are being developed that not only act as solvents but also as materials for particular applications (Ye, C. F., et al., Wear, 253, 579-584 (2002)). While there are about 300 organic solvents widely used in the chemical industry, there are potentially many more useful ionic liquids; (7) The unique charge properties allow easy construction of IL on preformed templates which could generate complex chemical selective films. In summary, IL's offer tremendous diversity in structural and chemical properties and their unique properties offer an excellent opportunity to design an array of chemically selective IL films and explore their application in pattern recognition for various analytes.
Many research groups are developing new materials and transducers for gas sensing with particular emphasis on optimizing interface properties among the gas phase, the sensitive materials and the transducer. For example, self-assembled monolayers (SAM) have been used to construct functional organic surfaces (Baker, G. A., et al., The Analyst, 130-800-808 (2005)). They have the advantage of being easily and reproducibly synthesized, and the analysis rate is typically fast since they do not need to penetrate through a diffusion barrier. The disadvantage of SAM is that the chemical selectivity depends only on the terminal groups, making the degree of chemical selectivity that can be engineered into simple SAM not as great as in thicker or more complex materials. Moreover, the total number of receptors incorporated in the film and thus the dynamic range and sensitivity of the sensor, is limited by the surface area of the substrate. In order to overcome the disadvantages of SAM, stepwise self-assembled bilayers were reported (Baker, G. A., et al., The Analyst, 130, 800-808 (2005)), which can produce films of complex molecules and molecular assemblies. However, self-assembled films of complex molecules and molecular assemblies are difficult to prepare.
Thin films made from ILs can perform well as sensor interfaces and provide additional control over selectivity and sensitivity when interacting with analytes in gas phase. Most organic solvents or vapors are soluble in ILs. Therefore, the partition process will reach equilibrium very fast after the sensor is exposed to the vapors. This ensures a fast response and excellent reversibility. At equilibrium, the distribution of organic vapors in the IL phase and the gas phase will depend on the partial pressure of the vapors so quantitative measurement is feasible. ILs have zero vapor pressure and work in a very large temperature range which is ideal for industrial high temperature sensing applications.
ILs possess high ion concentration, high heat capacity and good electrochemical stability. They prove to be excellent candidates for highly efficient heat transfer fluids, supporting media for catalysts as well as electrochemical devices including supercapacitors, fuel cells, lithium batteries, photovoltaic cells, electrochemical mechanical actuators and electroplating (Handy, S. T., Chem. Eur. J. 9 2938-2944 (2003); Ding, J., et al., Chem. Mater. 15 2392-2398 (2003: Jensen, M. P. et al., J. Am. Chem. Soc. 125 15466-15473 (2003); Yang, C., et al., J. Phys. Chem. B, 107 12981-12988 (2003: Barisci, J. N., et al., Electrochem. Commun. 6 22-27 (2004; Wang, P., et al., J. Phys. Chem. B, 107 13280-13285 (2003)). Recently, reports for the use of ILs as lubricants for steels joints (Liu, W. M., et al., Tribology Letters 13 81-85 (2002: Wang, H. Z., et al., Wear 256 44-48 (2004: and Ye, C. F., et al., Wear, 253 579-584 (2002: show that the ILs exhibits excellent friction-reduction, antiwear properties, both in air and in vacuum, which are superior to phosphazene and perfluoropolyether.
Identifying and correcting emissions from high-polluting vehicles requires small sensors working at high temperatures to monitor pollutants in exhaust gas or leaking fuels (Tsang et al., J. Phys. Chem. B, 2001, 105, 5737-5742; Kaltenpoth et al., Anal. Chem., 2003, 75, 4756-4765). High temperature gas sensing is conventionally achieved by using semi-conductive metal oxides, such as SnO2 and TiO2 (Dutta et al., J. Phys. Chem. B, 1999, 103, 4412-4422; Ikohura and Watson, The Stannic Oxide Gas Sensor, CRC Press: Boca Raton, Fla., 1994; Zhu et al., Anal. Chem., 2002, 74, 120-124). The resistance of metal oxides changes in the presence of organic vapors, CO or H2. It takes relatively a long time to reach equilibrium for the sorption of analytes from gas phase onto the metal oxides, especially for porous materials. The dependency of the resistance of the metal oxides on the vapor concentration is not linear, which reduces the accuracy of quantitative analysis (Simon et al. J. Comb. Chem., 2002, 4, 511-515). Some metal oxides work only at temperatures higher than a “switch on” value, e.g. >700° C. for SrTiO3(Hu et al., J. Phys. Chem. B, 2004, 108, 11214-11218; Wang et al., J. Am. Chem. Soc., 2003, 125, 16176-16177; Dutta et al., Chem. Mater., 2004, 16, 5198-5204).
Rubbery polymers with low glass transition temperatures (Tg) have been used as coatings for detection of nonpolar or weakly polar organic vapors (Grate et al., Anal. Chem., 1993, 65, 987A). The vapor sorption in rubbery polymers is reversible and equilibrium is attained rapidly (Grate et al., Anal. Chem., 1993, 65, 987A; (a) Jarrett and Finklea, Anal. Chem., 1999, 71, 353; (b) Shinar et al., Anal. Chem., 2000, 72, 5981; (c) Zellers et al., Anal. Chem., 1995, 67, 1092; (d) Patrash and Zellers, Anal. Chem., 1993, 65, 2055). However, the mechanical properties of rubbery polymers strongly depend upon temperature (U. W. Gedde, Polymer Physics, Kluwer Academic Publ., Doedrecht, Netherlands, 1999). Most polymer materials with low Tg are not stable at high temperatures. Therefore, applications of polymer materials for high temperature vapor sensing are limited. Furthermore, if the vapors cannot absorb on the materials, the large surface-area to volume ratio sensing materials, such as graphite ((a) Jarrett and Finklea, Anal. Chem., 1999, 71, 353; (b) Shinar et al., Anal. Chem., 2000, 72, 5981; (c) Zellers et al., Anal. Chem., 1995, 67, 1092; (d) Patrash and Zellers, Anal. Chem., 1993, 65, 2055) or oxides (Dutta et al., J. Phys. Chem. B, 1999, 103, 4412-4422; Ikohura and Watson, The Stannic Oxide Gas Sensor, CRC Press: Boca Raton, Fla., 1994; Zhu et al., Anal. Chem., 2002, 74, 120-124) would not work for high temperature gas sensing.
U.S. Pat. No. 4,236,893 to Rice, U.S. Pat. No. 4,242,096 to Oliveira et al., U.S. Pat. No. 4,246,344 to Silver III, U.S. Pat. No. 4,314,821 to Rice, U.S. Pat. No. 4,735,906 to Bastiaans, and U.S. Pat. No. 6,087,187 to Wiegland et al. each teach using a piezoelectric sensor for the detection of an analyte in a liquid sample. U.S. Patent Application Publication Nos. 2003/0077222, 2003/0073133, 2003/0072710, 2003/0068273, 2003/0053950, and 2003/0049204, all to Leyland-Jones, discloses immunosensors which in particular embodiments have antibodies, Fab fragments, or scFv polypeptides immobilized on the surface thereof.
U.S. Patent Application Nos. 2002/0094531 to Zenhausern teach sensing probes such as a QCM for detecting a biological analyte of interest in gaseous, vapor, or liquid forms. The sensing probes are coated with various materials, such as polymers, ion exchange resins, porous silicon, silanes, thiols, and oxides. However ionic liquids are not taught as a coating for the sensing probes.
U.S. Patent Application Nos. 2002/0142477 to Lewis et al. teach organic vapor measurement using a polymer-coated quartz crystal microbalance. The quartz crystal microbalance crystals are coated with polymers including poly (ethylene-co-vinyl acetate) with 25% acetate (PEVA) and poly(caprolactone) (PCL) polymer films.
There is a need for improved devices which rely upon ILs.